Steam turbine power plants of a variety of configurations are generally known in the art. According to a known embodiment, the steam plant comprises a boiler, which evaporates feedwater into main steam at high temperature and high pressure (superheated steam), and a turbine unit comprising one or more sections and stages through which the steam is gradually expanded at decreasing pressures in order to power an electric generator. One typical embodiment in the prior art also includes one or more feedwater heaters that use steam extracted from one or more turbine stages or sections (a portion of the main steam flow through the turbine) to preheat the feedwater supplied to the boiler and evaporated into steam, in order to improve the overall thermal efficiency of the plant.
In one commonly known embodiment the steam plant operates on a reheat cycle where, after partial expansion through a higher pressure turbine section, steam is returned to a separate section of the boiler (commonly designated the reheater) where it is resuperheated (or "reheated") before being delivered to the next lower pressure turbine section as reheat steam. Reheating, whereby the reheat steam is returned to the next lower pressure turbine section at a higher temperature and enthalpy, is a generally accepted practice in nearly all larger steam plants to improve thermal efficiency. However, steam turbine plants employing reheat cycles and feedwater preheating systems require additional, more complicated piping, instrumentation and other systems.
Also known in the art is the so-called combined cycle plant, which combines a gas-turbine cycle with a steam-turbine cycle, each to power an electric generator. The combined cycle plant allows gains in thermal efficiency that are much larger than to those that may be obtained from reheating in a reheat cycle steam turbine plant. Accordingly, it would be advantageous to convert a reheat cycle plant to a non-reheat combined cycle plant.
The conversion of a reheat cycle steam plant to a combined cycle plant allows the elimination of the complicated reheat system but still improving the thermal efficiency, while reducing costs and providing a plant that is much easier to operate than a combined cycle plant with a reheat cycle. Typically, the conversion of a steam plant to a combined cycle plant requires the elimination of the intricate feedwater preheater system, which also simplifies the operation of the plant.
The elimination of the feedwater preheater system, with its various feedwater preheat steam extractions, however, reduces the total mass flow of steam through the various turbine sections and stages. The reduction in steam mass flow in the converted combined cycle plant may not be compatible with design limitations of one or more turbine sections in the original reheat cycle plant. Thus the conversion of a reheat steam plant to operate as a non-reheat combined cycle plant may require the costly internal modification of the steam turbine system.
Accordingly, it would be advantageous to convert a reheat steam plant to a non-reheat combined cycle plant without requiring internal steam turbine modifications.